Gaseous Diffusion and Transport

Question 1

What is the partial pressure of a gas?

Answer 1

The hypothetical pressure a gas within a mixture would exert on the container if it it occupied it alone.

Question 3

What is PB and what is its value?

Answer 3

PH = atmospheric pressure = 101Kpa or 760mmHg

Question 4

What is fiO2?

Answer 4

Th fraction of inspired air that is oxygen = 21%

Question 5

How do you calculate between KPA and mmHg?

Answer 5

KPA x 7.5 = mmHg

mmHg / 7.5 = KPA

Question 6

What formula for atmospheric pressure do you get if you merge Dalton’s law with the partial pressure of the gases which constitute air?

Answer 6

PB = PO2 + PCO2 + PN2 + PH2O (water vapour)

Question 7

How can you use Dalton’s law and partial pressures to calculate PO2?

Answer 7

If you know FO2 and PB you can calculate PO2, as PO2 = FO2 x PB

FO2 = 21% = 0.21
PB = 101KPA 

21% of 101 = 21KPA = PO2

Question 8

What happens to PO2 as you ascend in altitude?

Answer 8

PO2 starts to decrease as PO2 = FO2 x PB.

This is due to PB decreasing as you ascend.

FO2 stays the same, the air is still 21% oxygen.

Question 9

What is meant by the solubility of the of a gas?

Answer 9

Th amount of the gas which will dissolve into the liquid for a given partial pressure of gas.

Question 10

What is important for the transfer of gas with regards to partial pressure and concentration gradient?

Answer 10

It is NOT concentration gradient dependant, but is partial pressure dependant.

Question 11

What does Henry’s law describe?

What is its formula?

Answer 11

It describes the content of a gas in a liquid as the relationship between the gases solubility and the partial pressure it is exerting.

Content of gas in a liquid = solubility x partial pressure

Question 12

What is the constant value which is the PH2O in the lungs?

How saturated is the air in the lungs?

Answer 12

6.3KPA or 47mmHg.

Air in the lungs is 100% saturated.

Question 13

Why is water vapour important to take into account?

Answer 13

Because it affects the PiO2.

PiO2 = the partial pressure of oxygen in the inspired air at the end of the trachea.

PH2O must be taken away from PB before PiO2 can be calculated.

Therefore (PB-6.3) x 0.21 = PiO2.

Question 14

Can PAO2 be directly measured?

Answer 14

No, we must use other methods based on the information we know.

Question 15

What is the name of the equation which helps calculate PAO2?

Answer 15

The alveolar gas equation.

Question 16

Explain the derivation of the alveolar gas equation.

Answer 16

We know that CO2 moves from the capillaries to the alveoli to replace Oxygen, which moves in the opposite direction from the alveoli to the capillaries.

The directly unmeasurable PAO2 can therefore be calculated as the PiO2 - PCO2.

As the exchange of O2 and CO2 is not directly 1:1 we have to use the respiratory quotient (R) which is (CO2 production/O2 consumption) and is usually = 0.8. More oxygen is consumed than CO2 produced.

Therefore PAO2 = PiO2 - (PCO2/2)

Question 17

What is a normal PAO2?

Answer 17

13.5KPA

Question 18

What is a normal PACO2?

Answer 18

5.3KPA

Question 19

What is a normal PAH2O?

Answer 19

6.3KPA

Question 20

What is the normal PCO2 in atmospheric air?

Answer 20

0KPA.

Question 21

Why does oxygen enter the capillaries from the alveoli? Use partial pressure as part of your answer.

Answer 21

The partial pressure of oxygen in the alveolus is greater than that in the artery, therefore diffusion occurs down this pressure gradient (NOT concentration).

Question 22

How far along the pulmonary capillaries does the PAO2 = PcO2?

NB: PcO2 = capillary partial pressure of oxygen.

Answer 22

About 1/3 of the way along.

Question 23

What X2 factors affect the rate of diffusion of the gases across the alveolar-capillary membrane?

What do these X2 factors combine to make?

Answer 23

1) area
2) thickness

These combine to make ‘transfer factor’ = DLg

Question 24

What formula is given for the rate of transfer of gas, taking into account the transfer factor and partial pressure gradient?

What other factor is included in this equation?

Answer 24

Rate of diffusion = DLg x (PAO2 - PcO2) x (diffusivity)

Question 25

What is Dalton’s law?

What is its formula?

Answer 25

The total pressure of of a mixture of gases within a container is the sum of the partial pressures of the individual gases.

P(total) = p1 + p2 + p3…

Question 26

Which gas diffuses fastest, CO2 or O2?

Why?

Answer 26

CO2 = x20 times faster than O2.

It has a higher molecular weight which slows it down, but is much more solvable which increases its diffusivity as diffusivity = solubility / (square root of molecular weight).

Question 27

What is the alveolar ventilation equation?

Answer 27

PAO2 and PaO2 ∝ (CO2 production/alveolar ventilation)

Therefore if ventilation halves (assuming no change in CO2 production rate), PAO2 and PaO2 double.

Vice Versa is also true.

Question 28

How many mls of oxygen can each gram of Hb combine with?

What is a normal blood Hb concentration?

How can we use these numbers to calculate the oxygen carrying capacity of the Hb in the blood?

Answer 28

1. 34ml
   150g. L^-1
2. 34 x 150 = 200ml.L^-1

Question 29

Why would an anaemia patient have a normal oxygen saturation?

Answer 29

Their PaO2 would be fine as their inspired gases are unaffected. The Hb would be fully saturated but there would be fewer Hb molecules therefore you would see the signs of anaemia with normal oxygen saturations.